We are investigating the chemistry and physiology of transmitter release from a photoreceptor. The receptors of the giant barnacle, Balanus nubilus, have been chosen because their cell bodies, terminal regions, and the cells postsynaptic to them, are accessible for intracellular recording. This system is of particular interest because, as with vetebrate rods and cones, the receptors conduct passively, and a hyperpolarization of the receptor leads to a depolarization of postsynaptic cells. We have traced the individual receptors into the supraesophageal ganglion and observed their terminal ramifications with the light and electron microscope. We have shown that these receptors are not electrically coupled; thus with simultaneous intracellular recordings from receptor soma and axon we have been able to show that the visual signal in the terminal regions can be accounted for by the unusually high resistance and the rectification properties of the receptor membrane. We can identify visually and record from ganglion cells which respond to light ON and OFF and appear to be directly innervated by the receptors. The interaction between receptor and postsynaptic cell is being studied.